1 /* -*- Mode: js; js-indent-level: 2; -*- */
3 * Copyright 2011 Mozilla Foundation and contributors
4 * Licensed under the New BSD license. See LICENSE or:
5 * http://opensource.org/licenses/BSD-3-Clause
7 * Based on the Base 64 VLQ implementation in Closure Compiler:
8 * https://code.google.com/p/closure-compiler/source/browse/trunk/src/com/google/debugging/sourcemap/Base64VLQ.java
10 * Copyright 2011 The Closure Compiler Authors. All rights reserved.
11 * Redistribution and use in source and binary forms, with or without
12 * modification, are permitted provided that the following conditions are
15 * * Redistributions of source code must retain the above copyright
16 * notice, this list of conditions and the following disclaimer.
17 * * Redistributions in binary form must reproduce the above
18 * copyright notice, this list of conditions and the following
19 * disclaimer in the documentation and/or other materials provided
20 * with the distribution.
21 * * Neither the name of Google Inc. nor the names of its
22 * contributors may be used to endorse or promote products derived
23 * from this software without specific prior written permission.
25 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
26 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
27 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
28 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
29 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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38 var base64 = require('./base64');
40 // A single base 64 digit can contain 6 bits of data. For the base 64 variable
41 // length quantities we use in the source map spec, the first bit is the sign,
42 // the next four bits are the actual value, and the 6th bit is the
43 // continuation bit. The continuation bit tells us whether there are more
44 // digits in this value following this digit.
52 var VLQ_BASE_SHIFT = 5;
55 var VLQ_BASE = 1 << VLQ_BASE_SHIFT;
58 var VLQ_BASE_MASK = VLQ_BASE - 1;
61 var VLQ_CONTINUATION_BIT = VLQ_BASE;
64 * Converts from a two-complement value to a value where the sign bit is
65 * placed in the least significant bit. For example, as decimals:
66 * 1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
67 * 2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
69 function toVLQSigned(aValue) {
71 ? ((-aValue) << 1) + 1
76 * Converts to a two-complement value from a value where the sign bit is
77 * placed in the least significant bit. For example, as decimals:
78 * 2 (10 binary) becomes 1, 3 (11 binary) becomes -1
79 * 4 (100 binary) becomes 2, 5 (101 binary) becomes -2
81 function fromVLQSigned(aValue) {
82 var isNegative = (aValue & 1) === 1;
83 var shifted = aValue >> 1;
90 * Returns the base 64 VLQ encoded value.
92 exports.encode = function base64VLQ_encode(aValue) {
96 var vlq = toVLQSigned(aValue);
99 digit = vlq & VLQ_BASE_MASK;
100 vlq >>>= VLQ_BASE_SHIFT;
102 // There are still more digits in this value, so we must make sure the
103 // continuation bit is marked.
104 digit |= VLQ_CONTINUATION_BIT;
106 encoded += base64.encode(digit);
113 * Decodes the next base 64 VLQ value from the given string and returns the
114 * value and the rest of the string via the out parameter.
116 exports.decode = function base64VLQ_decode(aStr, aIndex, aOutParam) {
117 var strLen = aStr.length;
120 var continuation, digit;
123 if (aIndex >= strLen) {
124 throw new Error("Expected more digits in base 64 VLQ value.");
127 digit = base64.decode(aStr.charCodeAt(aIndex++));
129 throw new Error("Invalid base64 digit: " + aStr.charAt(aIndex - 1));
132 continuation = !!(digit & VLQ_CONTINUATION_BIT);
133 digit &= VLQ_BASE_MASK;
134 result = result + (digit << shift);
135 shift += VLQ_BASE_SHIFT;
136 } while (continuation);
138 aOutParam.value = fromVLQSigned(result);
139 aOutParam.rest = aIndex;